1. Field of the Invention
The invention relates to a ventilated brake rotor.
2. Background Information
To decelerate a vehicle in motion or to maintain control of the vehicle""s speed on a downhill grade, the vehicle must dissipate energy. Most cars, trains, airplanes, elevators, and other machines employ friction brakes for this purpose. For each wheel of an automobile, conventional disc-type brakes include two opposing, moveable steel plates, each faced with a heat- and wear-resistant lining known as a brake pad. Between a pair of brake pads is a brake rotor that rotates as the axle of the automobile turns. The brake rotor provides two conversion surfaces, against which a pair of brake pads may be applied so as to slow a vehicle down or bring it to rest through controlled friction by variable application of force. The kinetic or potential energy absorbed by the controlled slippage is converted into heat by the controlled friction. The heat generated is transferred principally within the brake rotor.
The heat produced by the conversion of the kinetic or potential energy of a vehicle traveling at normal highway speeds is significant. Moreover, more than eighty percent of this heat is conducted into the brake rotor. Excessive temperature in the rotor has numerous adverse consequences, such as distortion of the rotor surfaces or a decrease in the frictional force between the brake pads and the brake rotor (known as fading). Therefore, a brake system design needs to promote cooling of the brake rotor.
One technique to promote cooling of the brake rotor is to increase the thickness of the brake rotor so as to increase heat capacity. However, in modern rotor designs, increasing heat capacity by increasing mass and therefore weight is undesirable with the common materials used such as aluminum or metal matrix composite and especially Grey cast iron. A more common technique to cool down the brake rotor is to employ a ventilated brake rotor.
A ventilated brake rotor includes two parallel annular discs or rings supported at a distance from one another by a series of vanes. These annular discs are radially defined by an internal surface located at a first radius from their center and an external surface located at a second radius from their center. The series of vanes defines flow passages or channels between each adjacent pair of vanes. These flow passages may be distributed between the annular discs so that, as the brake rotor turns about its axis, the vanes induce air flow from the internal surface of the brake rotor, through its interior, and out to the external surface of the brake rotor. The passing air draws heat from the brake rotor and expels it radially outward in the direction of rotor movement. This process transfers heat away from the brake rotor through convection and cools it down.
The field is full of ventilated brake rotor patents that claim to describe vane designs that define efficient flow paths for air cooling purposes. Examples include U.S. Pat. Nos. 5,878,848, 5,492,205, and 5,161,652. Contrary to the assertions made in these patent documents, they do not incorporate and often contradict modern turbo-machinery principles.
U.S. Pat. No. 5,878,848 is assigned to General Motors of Detroit, Mich. and is employed in the 1998 Chevrolet Corvette C-5 front brake rotor. U.S. Pat. No. 5,878,848 discloses a ventilated brake rotor that focuses on the included angle between the cord at the nose of a vane and the radial line drawn through that point. By employing a zero or low angle of attack, U.S. Pat. No. 5,878,848 hopes to minimize incident and viscous losses so as to maximize rotor cooling air flow and the cooling rate.
To achieve a low angle of attack, the vanes of U.S. Pat. No. 5,878,848 are defined about a curved line having a curvature that is defined from at least one point on the locus radius of the rotor. This results in each vane being perpendicular or nearly so to the annular disc cover inner diameter as represented by the tangential flow vector. However, a low angle of attack would be applicable only for high air flow rates and zero or low angular velocity or rotation velocity, such as would be the case if the incoming air stream were further pressurized by a supplemental blower or the vehicle were moving very slowly. Neither of these are practical solutions. Moreover, another problem with this vane design is that it leads to separation of the air flow at the leading edge of the vane, followed by reattachment of the flow downstream. This separation and reattachment results in an increase in the air flow losses and, therefore, a decrease in air flow. Moreover, the inlet appears to form flow separation bubbles and turbulence that reduce the actual opening size of the inlet and result in reduced airflow. The decrease of the air flow losses and reduced airflow decreases cooling of the rotor. When tested, the performance of an embodiment disclosed in U.S. Pat. No. 5,878,848 was inferior to an embodiment of U.S. Pat. No. 5,161,652.
U.S. Pat. No. 5,161,652 is assigned to Toyota of Tokyo, Japan and is employed in the 1998 Toyota Supra. U.S. Pat. No. 5,161,652 describes a ventilated brake rotor design that continuously accelerates the air flow through vanes having essentially parallel walls until the air flow is perpendicular or even countercurrent to the tangential air flow at the exit of the air flow passage. The problem with this is that the disclosed vanes produce flow separation on the trailing side of the vanes. U.S. Pat. No. 5,161,652 does, however, correctly identify the inlet attack angle range as between forty and fifty degrees.
Another document disclosing a ventilated brake rotor is U.S. Pat. No. 5,492,205. U.S. Pat. No. 5,492,205 is assigned to General Motors and was invented by the same inventor as U.S. Pat. No. 5,878,848. Disclosed within U.S. Pat. No. 5,492,205 is a process to determine the ideal number of vanes for a given diameter of disc. The disclosed process brings a hydraulic principle into the heat transfer and air flow field of ventilated brake rotors. The number of vanes is determined by calculating the number that would result in a square vent profile for maximum vent cross sectional area to minimum surface area. The problem is that U.S. Pat. No. 5,492,205 does not use methods such as thermal conductivity, fin or vane efficiency, and boundary layer analysis, including considerations of changes in direction, acceleration and diffusion.
U.S. Pat. No. 5,544,726 describes the need to consider, in low mass brake rotors, balanced heat conduction to minimize distortion. This concept was also disclosed in U.S. Pat. No. 4,865,167, but with respect to radially straight vane designs.
In a curved vane design, like in all the above patents except U.S. Pat. Nos. 5,492,205 and 4,865,167, there is a beneficial consequence that the curved vane design resists heat induced distortion. This is true for angle vane designs as well. In design of U.S. Pat. No. 5,544,726, the highest material cross section is at the lowest radial position, nearer to the center of the rotor in contrast to where the greatest amount of heat is generated and where the greatest heat conductive transfer rate is required, at the outside. U.S. Pat. No. 5,544,726 focuses on balanced heat conduction while ignoring the fact that convection by airflow is the controlling heat transfer mechanism in this system, especially if U.S. Pat. No. 5,544,726 is trying to minimize the conduction of heat into the surrounding assembly.
One problem with U.S. Pat. No. 5,544,726 is that the airflow is grossly restricted by the width of the vanes and resultant vent shape at the inlet. The airflow further is restricted by a secondary set of vanes. This secondary set of vanes will cause a flow blockage at the narrowest point of the vent passages during modest airflow velocities due to the large amounts of turbulence and separation bubbles formed in the wake trailing behind the secondary vane. U.S. Pat. No. 5,544,726 is not known to have been embodied in a production application to date.
It is known that the rate of heat transfer from a ventilated brake rotor is in a direct relationship to the velocity of the air in the vent by forced convection. See generally, Michael D. Hudson and Roland L. Ruhl, xe2x80x9cVentilated Brake Rotor Air Flow Investigationxe2x80x9d, SAE International Congress, Detroit, Mich., February 1997. However, none of the above inventions has sufficiently taken this into account. Thus, what is needed is an increase in the convection heat transfer rate between the brake rotor and the air. In particular, what is needed is an improvement in the air flow capacity of existing ventilated brake rotors.
A ventilated brake rotor having vents disposed between two discs. The vents are defined by an adjacent pair of vanes distributed around the first disc and disposed below the second disc. Each vane is defined by a camber. The camber, in turn, is parallel to a curve that is defined by two camber lines. The curve is tangent to the leading end of a first camber line and tangent to a trailing end of the second camber line. The first and the second camber lines intersect where the first camber line is defined from a point on the interior radius of one of the two discs that intersects a datum. Moreover, the first camber line extends at a throat angle from the datum.